With current annual production at over 600 million tonnes, wheat is the third largest crop in the world behind corn and rice, and an essential source of carbohydrates for millions of people. While wheat is grown over a wide range of environments, it is common in the major wheat-producing countries for grain filling to occur when soil moisture is declining and temperature is increasing. Average global temperatures have increased over the last decades and are predicted to continue rising, along with a greater frequency of extremely hot days. Such events have already been reported for major wheat growing regions in the world. However, the direct impact of past temperature variability and changes in averages and extremes on wheat production has not been quantified. Attributing changes in observed yields over recent decades to a single factor such as temperature is not possible due to the confounding effects of other factors. By using simulation modelling, we were able to separate the impact of temperature from other factors and show that the effect of temperature on wheat production has been underestimated. Surprisingly, observed variations in average growing-season temperatures of ±2 °C in the main wheat growing regions of Australia can cause reductions in grain production of up to 50%. Most of this can be attributed to increased leaf senescence as a result of temperatures >34 °C. Temperature conditions during grain filling in the major wheat growing regions of the world are similar to the Australian conditions during grain filling. With average temperatures and the frequency of heat events projected to increase world-wide with global warming, yield reductions due to higher temperatures during the important grain-filling stage alone could substantially undermine future global food security. Adaptation strategies need to be considered now to prevent substantial yield losses in wheat from increasing future heat stress.